Do I have Chronic Kidney Disease (CKD) stage 3a with an estimated Glomerular Filtration Rate (eGFR) of 58 and no kidney damage, or is my kidney function better based on a Cystatin C test indicating an eGFR of 70?

Understanding Your CKD Status Based on eGFR and Cystatin C Results

Direct Answer to Your Question

You likely do NOT have CKD stage 3a, and your kidney function is better than suggested by the creatinine-based eGFR of 58. The cystatin C value of 1.04 mg/L with an eGFR of 70 indicates your true kidney function is likely in the normal to mildly reduced range, not stage 3a CKD. 1

Critical Clarification About Your Test Results

Your assumption about the eGFR of 70 being cystatin C-based is correct. When laboratories report cystatin C results, they typically include the cystatin C-based eGFR (eGFRcys) calculated from that value. 1 However, there's an important issue with your cystatin C level:

• A cystatin C value of 1.04 mg/L is actually slightly elevated, not normal. The 2012 CKD-EPI cystatin C equation uses 0.8 mg/L as the reference point, meaning values above this indicate some degree of kidney function reduction. 1
• Despite this elevation, your eGFRcys of 70 mL/min/1.73 m² still indicates better kidney function than the creatinine-based eGFR of 58. 1

Understanding CKD Diagnosis Requirements

CKD cannot be diagnosed by eGFR alone—you must have BOTH reduced eGFR AND evidence of kidney damage. 2 The key distinction is:

• CKD Stage 1-2: eGFR ≥60 mL/min/1.73 m² WITH kidney damage (albuminuria, abnormal imaging, or pathological findings) 2
• CKD Stage 3a: eGFR 45-59 mL/min/1.73 m² with OR without kidney damage 2
• Kidney damage is defined as: UACR ≥30 mg/g or abnormalities in pathological, urine, blood, or imaging examinations 2

If you have no proteinuria (normal UACR) and no other evidence of kidney damage, you did NOT have CKD stage 2 when your eGFR was in the 60s. 2

The Discordance Between Your Two eGFR Values

When creatinine-based and cystatin C-based eGFR values differ by more than 20% (as yours do: 58 vs. 70), you should use the combined creatinine-cystatin C equation (eGFRcreat-cys) for the most accurate assessment. 1, 3

• The combined equation demonstrates 94.9% accuracy within 30% of measured GFR 1, 3
• In cases of discordance, the combined equation shows 84% accuracy versus only 50% for creatinine alone and 73% for cystatin C alone 1
• You should request that your laboratory calculate the eGFRcreat-cys using both your creatinine and cystatin C values together. 1, 3

Why Your eGFR Values Differ

The discordance between your creatinine-based eGFR (58) and cystatin C-based eGFR (70) suggests that creatinine may be overestimating your kidney function decline. This commonly occurs with:

• Variable or reduced muscle mass 3
• Malnutrition 1
• Extremes of body composition 1
• Aging-related muscle loss 4

Creatinine is produced by muscle breakdown, so lower muscle mass leads to lower creatinine levels independent of kidney function, which paradoxically makes the eGFR appear worse than it actually is. 3

Clinical Significance of the Difference

A large positive difference between eGFRcreat and eGFRcys (where creatinine-based is lower) may indicate cardiovascular risk, but in your case the difference goes the opposite direction. 5 Your situation—where cystatin C-based eGFR is higher—suggests the creatinine-based estimate is less accurate for you specifically. 1, 3

Recommended Next Steps

Request the following from your healthcare provider: 1, 3

1. Calculate the combined eGFRcreat-cys using the 2012 CKD-EPI creatinine-cystatin C equation with both your values
2. Verify you have no proteinuria by checking UACR (urinary albumin-to-creatinine ratio)
3. Repeat both creatinine and cystatin C measurements in 3 months to confirm stability, as CKD diagnosis requires abnormalities persisting for >3 months 6

Important Caveats About Single Measurements

Using a single eGFR measurement overestimates CKD prevalence by approximately 25% compared to using two measurements separated by at least 3 months. 6 This is why current guidance recommends two positive tests before confirming CKD diagnosis. 6

There is inherent uncertainty in eGFR measurements at the critical 60 mL/min/1.73 m² threshold. 4 The uncertainty at this decision level is calculated to be ±11 mL/min/1.73 m², meaning caution is needed when interpreting eGFR values between 49 and 71. 4 Your values fall squarely in this uncertain range.

Bottom Line for Your Situation

Based on the available evidence, you most likely have normal kidney function or at most mild reduction (not stage 3a CKD), especially if you have no proteinuria or other kidney damage. 1, 3 The combined equation will provide the most accurate assessment, and repeat testing in 3 months will confirm whether any reduction is persistent or transient. 6, 7